3D Petro‐Structural Evolution of the High Pressure‐Low Temperature Phyllite‐Quartzite Nappe Pile in Southern Peloponnese, Greece

Abstract Tectonic underplating of high‐pressure/low‐temperature (HP‐LT) tectonic slices is a key mechanism in crustal growth at convergent margins. Yet, the processes controlling the geometry, depth and sequence of underplating events remain poorly constrained. We investigate the 3D petro‐structural architecture of the Phyllite‐Quartzite (PQ) nappe stack in southeastern Peloponnese (Greece), a well‐preserved segment of an Oligo‐Miocene accretionary complex within the Hellenic subduction zone, to constrain how successive pluri‐kilometric slices were formed and exhumed. Detailed structural mapping, petrology, Raman thermometry and thermobarometric modeling reveal two distinct sub‐units: Velanidhia and Pandanassa, interpreted as individual tectonic slices underplated at ∼50–60 km depth. Their pressure peak conditions (Velanidhia: 16 ± 1 kbar, 480 ± 10°C; Pandanassa: 15–18 kbar, 435 ± 21°C) and contrasted retrograde pressure‐temperature paths, witnessing post‐peak heating in Velanidhia and isothermal decompression in Pandanassa, reflect discrete underplating events under evolving thermal regimes. Geological cross‐sections and structural analysis reveal a deep paleo‐accretionary wedge geometry, involving a top‐to‐the‐E ductile‐brittle detachment that accommodated syn‐orogenic exhumation of a dome‐shaped HP‐LT nappe stack later crosscut by steep normal faults. Thanks to the detailed characterization and structural architecture of the PQ series, it is possible to constrain the trench‐parallel (≥115 km) and trench‐perpendicular (≥50 km) dimensions of each tectono‐metamorphic unit, confirming their pluri‐kilometric lateral extent. This work provides new insights into how episodic underplating at the subduction interface, witnessed by stacked tectono‐metamorphic units with contrasted P‐T evolution, governs the architecture of paleo‐accretionary wedges at active margins.